At one time individual active and passive components were interconnected on a surface of a single circuit board by metal leads, wires and conductive traces. Today, as electronic systems require greater speed and complexity in smaller and smaller packages, printed circuit designers have been encouraged to develop printed circuit boards having greater circuit densities. Much of the early advances in printed circuit technology involved reductions in scale, such as by decreasing the line thicknesses for conductive traces, combining multiple functions on integrated circuit chips, and directly mounting integrated circuit chips through leaded and surface mounting processes.
Further development of printed circuit technology involved the creation of double-sided circuit boards and multilayered boards. These circuit boards typically comprise two or more conductive layers fabricated on one or more insulating substrate layers. The circuit layers are then connected by means of conductors (plated through holes) passing through the insulating layers. While early double-sided boards used insulating layers made of a rigid resin or ceramic material, many printed circuit boards in use today employ flexible substrates, typically made of a polyester or polyimide material.
However, a circuit board needs to be connected to other boards or electronic devices, power supply, etc. Initially such connections were done by point to point wiring. As the number of connections in a circuit board that need to be connected to points outside the board increase, more sophisticated interconnectors were needed. Devices such as ribbon wire connectors were utilized for such connections. In a system with many circuit boards and interconnectors for connecting the circuit boards, it is desirable for the interconnectors to be flexible. The flexibility of the interconnectors improves reliability of the interconnected circuit boards and facilitates the handling of the boards during installation and repair. These flexible interconnectors lower the risk of incorrect wiring and reduce damages on the contacting points between the interconnectors and the circuit boards due to movement and vibration. Typically, interconnectors such as ribbon wire connectors are connected to circuit boards by soldered connectors, sockets and pins, wire wraps plug-ins, zero insertion force connectors, or by conventional soldering techniques. Rather than using wires or ribbon wire connectors to connect circuit boards, a flexible circuit board can be used as the interconnector. One advantage of using such a flexible circuit board interconnector is the possibility of mounting components directly on the flexible circuit board interconnector, further increasing the circuit and component density in limited space. Other advantages are added flexibility, durability, flatness and low profile.
Flat flexible interconnectors, circuit or jumpers can be connected to circuit boards with methods of connecting layers in a multilayer circuit board. However, many of conventional methods of making multilayer circuit boards are expensive and may not be well suited for making rigid-flexible circuit boards that are able to tolerate mechanical strain, thermal cycling, vibration, and movement. There is a need for a reliable mechanical sturdy flexible interconnector for interconnecting rigid circuit boards.
There are many ways to connect circuit boards. Circuit boards in use today can be single-sided, double-sided, single-layered or multilayered. In conventional multilayer circuit fabrication, the two circuit layers on a double-sided circuit board are typically connected by the plated-through hole (PTH) method, which involves fabricating holes through the insulating layer and forming a layer of plated copper along the barrel surface of the hole to electrically connect both layers.
In multilayer printed circuit boards having two or more interconnected conductive layers, other techniques are needed to securely connect the conductive layers. One attempt to overcome the problems associated with PTH processes is shown in U.S. Pat. No. 3,795,047, issued to Abolafia et al. This reference describes a multilayer electronic circuit having limited areas of metal powder and epoxy selectively applied at electrical connection points between two conductive layers.
Due to a number of problems associated with Abolafia et al., high density multilayer circuit boards may not be reliably produced with this method. The method of applying the metal particles at selective locations on the circuit board presents a number of problems. Problems with the process include: uneven distribution of the particles and the need for the epoxy to be selectively applied to different points along the circuit board.
Finally, the process of Abolafia et al., while being conceptually simpler than the PTH process, still requires a number of steps to adequately secure the boards together. The reference requires selective registration of adhesive, application of metal powder, removal of excess metal powder, additional registration of adhesive along the other surface, and finally, alignment, pressing and heating to create the full assembly.
There are a few reference related to z-axis conductive adhesives. U.K. Patent Application No. 2,068,645 and French Patent Application No. 2,475,302 describe such an adhesive consisting of a number of silver coated glass spheres disposed within a thermoplastic material. The conductive particles are sized such that one or two of them is sufficient to bridge the gap between opposed planar conductors.
Likewise, European Patent Application No. 265,212 entitled "Electroconductive Particles and Electroconductive Adhesive Containing Said Particles" discloses conductive particles which are fine polymer particles with a thin metal layer disposed on their surfaces.
The above references are similar in the fact that the conductive particles are non-deformable. This results in connections being formed and maintained by the pressure exerted by the adhesive in holding the conductive layers together. Connections formed with this pressure-acting method are, in general, only moderately reliable, and may be prone to failure, especially during thermal cycling.
European Patent Application No. 147,856 entitled "Electrically Conductive Adhesive Sheet, Circuit Board and Electrical Connection Structure Using the Same", discloses an electrically conductive adhesive sheet having a number of electrically conductive metal particles disposed within an insulating adhesive. During a heat and pressure applying step, these particles are "squashed" between the conductive layers, and reflowed to form fused connections.
However, these conductive adhesives are unable to withstand the temperatures required for component assembly. This is because these adhesives are used after a circuit board has been component assembled. Much of the high temperature soldering processes have already been applied prior to the application of the adhesives. Therefore, they are not developed for the purpose of having heat resistant properties.
European Patent Application No. 346,525 entitled "Multilayer Electronic Circuit and Method of Manufacture," discloses a multilayer electronic circuit comprising three or more electronic circuit layers connected by an interconnector layer which contains fusible solder particles. The quantity of the particles is selected to achieve this end.
However, the conductive adhesives above do not take into account for thermal expansion, as the applications for which they are used do not typically involve high or low temperatures. Due to a mismatch in thermal coefficients of expansion (TCE) between the adhesives and the insulating substrates or films which are being connected, the connections created by these conductive adhesives may inadvertently open during thermal cycling. Finally, the adhesives themselves may not be temperature resistant, and may soften or lose their bond ply under excessive temperatures. This problem is especially prevalent in systems which undergoes repeated thermal cycling, movement, and vibration, such as in equipment used in vehicles for transportation, for example, in automobile.
Some printed circuits containing multiple layers have both rigid and flexible portions. Dixon et al., U.S. Pat. No. 4,800,461, describes multilayer rigid flex printed circuits having isolator materials which when subjected to elevated temperatures, do not expand sufficiently in the z direction to cause delamination. The multilayer rigid flex printed circuit construction contained an adhesive, and electrical conduction between the layers is provided by plate through barrels. Desai, U.S. Pat. No. 5,004,639, describes a multilayer flexible circuit. Some of the rigid layers are attached to the flexible layers by an adhesive. Electrical conduction between the layers is provided by plate through holes. DeMaso et al., U.S. Pat. No. 5,072,074, discloses a printed circuit containing multiple layers and rigid and flexible portions. The multilayer circuit contains a sheet of flexible substrate material extending over the entirety of both the rigid and flexible portions, and sheets of a rigid substrate material extending over the entirety of all of the rigid portions. A flexible adhesive material attaches a heat of flexible overlay material to the entirety of all of the flexible portions and a rigid adhesive material attaches the sheets of a rigid substrate material to the entirety of all the rigid portions. Electrical conduction through the layers is by means of plate through vias. McKenney et al., U.S. Pat. Nos. 5,095,628 and 5,175,047 describe a rigid-flex printed circuit board with a rigid insulating layer supporting one portion of the printed circuit and a flexible insulator supporting another portion of the printed circuit. The means of electrical conduction between the electrical conductive layers is not clearly described.
Attempts have been made to use flexible circuit boards to connect rigid circuit boards. Morris et al., U.S. Pat. No. 4,064,622 is related to a flexible jumper strip for connecting to printed circuits. The flexible jumper strip has conductor leads that are made into pins with a v shape. The pins are connected to printed circuit by inserting into connector holes. Dixon et al., U.S. Pat. No. 4,064,357 describe a method of electrically interconnected layers of printed circuitry. An insulator is coated on both sides with an adhesive. Holes are punched through the insulator at points wherein the connections are to be made. Printed circuit patterns are formed on both sides of the insulator. Electrical conduction between the conductive circuit patterns is by means of solder bridges formed in the interconnecting holes. Porter et al., U.S. Pat. No. 4,928,206 discuss using foldable flexible circuit panels for connecting rigid printed circuit boards to facilitate cryogenic cooling in a liquid heat sink. However, they do not describe the method of connecting the rigid boards and the flexible circuit panels in detail. Knight et al., U.S. Pat. No. 5,102,343 describe an electrical connector actuated by fluid pressure in an expandable bladder, wherein the expanding bladder exerts force against the contacts on a flexible circuit to achieve electrical connection between rigid circuit members such as printed boards. However, in none of these references was the application of an anisotropic conductive adhesive suitable for making multilayered rigid-flex circuit boards that can tolerate thermal cycling and mechanical challenge disclosed.
U.S. patent application Ser. No. 08/001,811 filed on Jan. 8, 1993 by Keith L. Casson et al. describes a multilayered circuit board containing flexible layers having an anisotropic conductive adhesive layer disposed between the circuit layers. U.S. patent application Ser. No. 08/002,177 filed on Jan. 8, 1993 by Keith L. Casson et al. describes a multilayered circuit suitable for application in a transportation environment. These two U.S. patent applications are incorporated by reference herein.
There is a need for a rigid-flex circuit board having at least one rigid board connected to at least one flexible interconnector that can be bent such that the rigid-flex board can be positioned to utilize space efficiently. The interconnection between the boards and the flexible interconnector should be capable of withstanding the stress and strain involved in the construction and final assembly at the point of use and thermal cycling and vibration. Furthermore, a need exists for a method of making such rigid-flexible circuit boards which is less complex than the prior PTH processes and which produces reliable interconnections between conductive layers. Because in certain instances rigid-flex circuit board assemblies may need to be further processed, there is further a need for a multilayer circuit made with an anisotropic conductive adhesive that is heat resistant, i.e. able to tolerate soldering where electronic components need to be affixed.